Tunable filter device for spatial positioning systems

ABSTRACT

A device for spatial positioning systems includes a programmable measurement filter that is dynamically tuned by the device signal processing unit. The signal processing unit analyzes available data including the measurement signal to determine the likelihood that a detected measurement comprises true device movement and adjusts the measurement filter bandwidth accordingly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/341,571 filed Jan. 13, 2003.

BACKGROUND OF THE INVENTION

The present invention relates in general to devices such as receiversand sensors for spatial positioning systems and in particular to devicesfor spatial positioning systems having tunable measurement filters.

Spatial positioning systems provide a convenient tool for takingaccurate measurements such as distance, angle, or position. Basically,one or more transmitters emit signals that are detected by a receiverdeployed about a work site. The receiver processes the emitted signalsand determines a measurement therefrom. Alternatively, some devices forspatial positioning systems, such as certain tilt sensors, derivemeasurements without the need for a separate transmitter. In eitherrespect, the determined measurement signals are typically output to adisplay or provided as a feedback signal to a control system. Theflexibility and accuracy of spatial positioning systems has made suchsystems suitable for use in a number of diverse applications includingfor example, building and general construction, earthmoving, surveying,navigating, vessel and structure placement and other applications whereit is desirable to accurately take measurements.

Presently, receivers for spatial positioning systems provide signalconditioning and filtering to improve the reliability of measurementstaken thereby. However, the receiver filter operates on a ‘one-size’fits all approach regardless of application or environment. While fixedfilter receivers may be satisfactory for removing noise under someconditions, there are situations where a fixed filter receiver may notprovide optimal results. For example, operating environments may havedrastically different and dynamically changing noise levels due to beambounce, electrical interference, weather conditions, such as gusts ofwind, and operational conditions, such as vibration and operatorhandling. A receiver having a filter tuned properly to filter out noisedue to a relatively high frequency vibration may be ineffective atfiltering relatively low frequency noise resulting from beam bounce.However, a filter that is suitable for filtering relatively lowfrequency noise, such as produced by beam bounce, may have excessive lagthat makes the processing delay due to the filter performanceimpractical, such as for real-time control operations.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of previously knowndevices for spatial positioning systems by providing a device having aprogrammable measurement filter that is automatically and dynamicallytuned during operation. The measurement filter is dynamically tuned forexample, to enhance measurement performance by reducing noise, jitterand other undesirable signals that would otherwise adversely affect ameasurement taken by the device. The device is further preferablycapable of manipulating a measurement taken by the measurement filter toproduce a suitably formatted measurement output signal, whether themeasurement output signal is intended for a display or an automatedcontrol application communicably coupled to the device.

According to an embodiment of the present invention, a programmablemeasurement filter is dynamically and automatically tuned to behave as afilter having a relatively high bandwidth to allow quick responsetracking of true device movements and as a relatively lower bandwidthfilter to attenuate noisy feedback. For example, the programmablemeasurement filter is automatically tuned to implement a low pass filterfunction having a first relatively high cutoff frequency when the devicedetects true device movement. The programmable measurement filter isautomatically tuned to implement a low pass filter function having asecond, relatively lower cutoff frequency to attenuate noise.

According to another embodiment of the present invention, a device isprovided having a programmable measurement filter that is dynamicallyand automatically tuned to behave as a filter having a predeterminedmaximum bandwidth to allow quick response tracking of true devicemovements, a filter having a predetermined minimum bandwidth filter toattenuate noisy feedback, and as a filter having a bandwidth somewherebetween the maximum bandwidth and the minimum bandwidth depending uponpredetermined conditions. Tuning between the maximum and minimumbandwidths may be accomplished either continuously or in discrete steps.Tuning of the measurement filter may be accomplished based upon anydesired criteria. For example, the programmable measurement filter maybe programmed to have a filter bandwidth that varies somewhere between afilter bandwidth minimum and maximum depending upon the likelihood thattrue device movement is being measured compared to noise.

According to an embodiment of the present invention, a deviceautomatically and dynamically filters a measurement signal using atwo-pole IIR (infinite impulse response) digital filter that is tuned insoftware to produce a signal suitable for display output and/or forcontrol systems. By analyzing available data, the device tunes thefilter to present a lower or higher bandwidth system within somepre-defined range of operation depending upon the application and needsof the device operator.

According to yet another embodiment of the present invention, a deviceincludes a programmable measurement filter that is dynamically andautomatically tuned during operation. By analyzing available data, atuning signal is generated. The tuning signal is used to automaticallytune the programmable measurement filter. The tuning signal may befurther provided for example, as a feedback signal to a control system.Alternatively, both the tuning signal and a measurement signal filteredby the programmable measurement filter may be provided as feedbacksignals to a control system.

The programmable measurement filter according to various embodiments ofthe present invention may be adapted to filter any spatial positioningmeasurement signal including for example, angular measurements,acceleration measurements, detected laser signals, detected globalpositioning signals and detected automated tracking system signals.Also, the programmable measurement filter may be implemented in eitherthe analog or digital domains. For example, the programmable measurementfilter may be implemented digitally in software executable by a signalprocessing unit of the device. Further, the programmable measurementfilter output may be coupled either to a display device or to a controldevice. The display or control unit may be integral with the device, oralternatively, the device may communicate an output based upon thefiltered measurement signal to a remote display or control unit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of the preferred embodiments of thepresent invention can be best understood when read in conjunction withthe following drawings, where like structure is indicated with likereference numerals, and in which:

FIG. 1 is a front view of an exemplary device for a spatial positioningsystem;

FIG. 2A is a block diagram of a device according to an embodiment of thepresent invention;

FIG. 2B is a block diagram of a device according to an embodiment of thepresent invention illustrating a detector, signal processing unit andoutput device coupled together in an integrated housing;

FIG. 2C is a block diagram of a device according to an embodiment of thepresent invention illustrating a detector and signal processing unitincorporated into a housing that is communicably coupled to a remotelypositioned output device;

FIG. 2D is a block diagram of a device according to an embodiment of thepresent invention illustrating a signal processing unit and outputdevice incorporated into a housing that is communicably coupled to aremotely positioned detector;

FIG. 2E is a block diagram of a device according to an embodiment of thepresent invention illustrating a detector and an output deviceincorporated into a housing that is communicably coupled to a remotelypositioned signal processing unit;

FIG. 2F is a block diagram of a device according to an embodiment of thepresent invention illustrating a detector, a signal processing unit andan output device, each remotely positioned and communicably coupledtogether;

FIG. 2G is a block diagram of a device according to an embodiment of thepresent invention illustrating a variety of detector configurations andoutput configurations;

FIG. 3 is a block diagram of a device according to an embodiment of thepresent invention;

FIG. 4 is a flow chart illustrating a method for automatically tuning adevice filter according to an embodiment of the present invention;

FIG. 5 is a flow chart illustrating a method for automatically tuning adevice filter according to another embodiment of the present invention;

FIG. 6 is a flow chart illustrating an approach to automatically tuninga device filter according to an embodiment of the present invention;

FIG. 7 is a block diagram illustrating an approach to automaticallytuning a device filter according to the flow chart of FIG. 6; and

FIG. 8 is a block diagram of a control application capable of adaptingto changes in phase lag according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration, and not by way oflimitation, specific preferred embodiments of the invention. It is to beunderstood that other embodiments may be utilized and that logical,mechanical and electrical changes may be made without departing from thespirit and scope of the present invention.

Referring to FIG. 1, an exemplary device 10 for a spatial positioningsystem according to an embodiment of the present invention isillustrated. The device 10 includes a housing 12, a detector 14, one ormore display devices 16, optional indicia 18 and one or more optionalinput devices 20. The detector 14 typically comprises one or moresensors adapted to sense a signal emitted by a correspondingtransmitter. The display devices 16 can include any number of visualindicators such as light emitting diodes (LEDs), liquid crystal displays(LCDs) or other screen based displays configured to convey measurementinformation determined by the device to the user. The term displaydevice is further to be interpreted broadly to include an audibleindicator, as well. For example, the device 10 may include a speaker orother sound producing device adapted to convey measurement informationaudibly. Although the display devices 16 are illustrated as integralwith the housing 12 in FIG. 1, the display devices may be optionalaltogether, or may be remotely coupled to the device 10, such as may bedesirable for remote operation. The indicia 18 is provided for example,to display scale markings, instructions or other information to a user.Also, the optional input devices 20 may include switches, buttons,knobs, sliders or other devices provided to allow an operator to inputoperational information or instructions into the device 10. The device10 may also optionally include connectors and mounting hardware tointerface the device with external devices including for example, rods,displays and control devices (not shown).

Referring to FIG. 2A, a basic block diagram 22 of a device according toan embodiment of the present invention is illustrated. The detector 14detects and outputs a measurement signal 24 that is communicably coupledto a signal processing unit 26. The signal processing unit 26 analyzesand processes the measurement signal 24 and produces an output signal 28that is communicably coupled to an output device 30. Referring to FIG.2B, the device 10 according to an embodiment of the present inventionincludes the detector 14 adapted to communicably couple the measurementsignal 24 to the signal processing unit 26. The signal processing unit26 is adapted to communicably couple the output signal 28 to the outputdevice 30. The detector 14, signal processing unit 26 and output deviceare arranged as an integral unit, for example, in a housing 12 such asthat described with reference to FIG. 1.

When referring to the expression “communicably coupled” herein, it ismeant that a signal is either directly connected between the source anddestination, or the signal may be optionally coupled through one or moreintermediate processes. For example, the measurement signal 24 output bythe detector 14 may be directly connected to the signal processing unit26, or the measurement signal 24 may be processed by intermediatehardware and/or software prior to reaching the signal processing unit26. As such, the signal processing unit 26 may actually receive amodified version of the measurement signal 24. For example, themeasurement signal 24 may be passed through an analog to digitalconverter, buffer, current or voltage converter and/or other signalprocessing hardware or software.

Further, the term communicably coupled is to be interpreted expansivelyto include any manner of communicating information includingunidirectional and/or bidirectional communication. For example,communication may be accomplished by direct electrical or opticalconnection, coupling through induction, wired or wireless links such asa radio link, serial link or other communications protocols. Forexample, the measurement signal 24 output by the detector 14 may becoupled to a radio link or other wireless communications device. Undersuch an arrangement, the signal processing unit 26 will likely includethe necessary capability to receive the measurement signal 24transmitted at the detector 14. Alternatively, the measurement signal 24may be processed by signal conditioning electronics in hardware orsoftware prior to reaching the signal processing unit 26. Thus themeasurement signal 24 generically refers to the output of the detector14 either directly, or as modified by intermediate processes prior tobeing filtered by the measurement filter.

Referring to FIG. 2C, a device 10 according to one embodiment of thepresent invention includes a detector 14 adapted to communicably couplethe measurement signal 24 to the signal processing unit 26 where thedetector 14 and signal processing unit 26 are provided in an integratedassembly. The signal processing unit 26 communicably couples the outputsignal 28 to a remotely positioned output device 30, such as by a wiredor wireless communications link. Referring to FIG. 2D, the device 10according to an embodiment of the present invention may alternativelyinclude the signal processing unit 10 and the output device 30 in anintegrated assembly. The detector 14 is remotely positioned relative tothe device 10 and is adapted to communicate the measurement signal 24 tothe device, such as through a wired or wireless communications link.

Referring to FIG. 2E, the device 10 according to an embodiment of thepresent invention is arranged such that the detector 14 and outputdevice 30 are both remotely positioned from the signal processing unit26. For example, as shown, the detector 14 and output device 30 arearranged together in an integral assembly. The measurement signal 24 andoutput signal 28 are communicated back and forth between the devices.Referring to FIG. 2F, the device according to an embodiment of thepresent invention provides the detector 14, the signal processing unit26 and the output device 30 each remotely located from one another.

The configuration of the detector 14 will depend upon the type ofmeasurement signals to be processed by the signal processing unit 26.Four illustrative detectors are shown in FIG. 2G, however, any one ormore of the illustrated detectors may be implemented in a particulardevice 10 according to the present invention. Also, detector types otherthan those illustrated may be used with the various embodiments of thepresent invention. For example, the detector 14 may include a lasersensor 14A, which typically comprises a photodiode or photodiode array.The laser sensor 14A is configured to sense and provide angle orpositional measurements detected from a laser transmitter. The detectedlaser signals are typically processed by the signal processing unit 26to generate a one-dimensional Z-coordinate measurement.

As another example, the detector 14 may comprise a GPS sensor 14Bconfigured to sense and provide positional measurements in up to threedimensions, such as providing X, Y, and Z-coordinate measurementsderived from a global positioning satellite system. Another exampleincludes a Total Station sensor 14C that is configured to sense andprovide positional measurements in up to three dimensions, such asproviding X, Y, and Z-coordinate measurements derived from a TotalStation system. Still further, the detector may comprise a tilt sensor14D configured to sense and provide acceleration or angular information(θ). The tilt sensor 14D can detect tilt information from an externallytransmitted source, or the tilt sensor may be adapted to detect tilt ofthe device itself without the need for a separate transmitter.

The signal processing unit 26 provides the control electronics necessaryto implement the desired functionality of the device. The signalprocessing unit generates the output signal 28 by processing themeasurement signal 24 provided by the detector 14. The signal processingunit 26 may also optionally include or implement signal buffers,amplifiers, integrators, sample and hold circuits and other necessarycircuits to condition the measurements 24. The signal processing unit 26also carries out and/or controls the adaptive filtering of themeasurement signal 24 via a programmable measurement filter 32. Thesignal processing unit 26 can include for example, any combination ofanalog and digital circuitry to implement the programmable measurementfilter 32 and corresponding support logic.

The measurement signal 24 is processed by the programmable measurementfilter 32 to produce a filtered measurement signal that is used by thesignal processing unit 26 to produce the output signal 28. In generatingthe output signal 28, the signal processing unit analyzes available dataincluding the measurement signal 24 to determine at least one filterparameter value therefrom. The programmable measurement filter 32 isdynamically programmed or tuned by the signal processing unit 26 basedupon the determined filter parameter value(s) to achieve improvedstability of the measurement signal 24. For example, for displayapplications, a more stable image is achieved. In control applications,a more stable control signal may be realized.

The type of output device 30 can vary depending upon the implementationof the device 10 and on the type of output signal 28 provided by thesignal processing unit 26. Likewise, the output signal 28 will beadapted for the appropriate output device. For example, the signalprocessing unit 26 may manipulate the measurement signal 24 filtered bythe programmable measurement filter 32 to generate the output signal 28in a format suitable for a display 30A. The display 30A can be orientedintegral with the device such as shown in FIG. 2B or the display 30A maybe separately located from the device such as shown in FIG. 2C. Thenature of the display will vary depending upon the type of measurementsignal being processed. For example, a laser device for the constructionindustry may include a display that includes a scale for indicating thedegree to which the device 10 is above or below the grade defined by alaser plane. A display for a GPS system may provide an alpha-numericreadout etc.

The output signal 28 may also provide information formatted suitably foruse as a control or feedback signal for automated and semi-automatedcontrol systems. For example, the output signal 28 can be used with ahydraulic control 30B to operate or control hydraulic valves. The outputsignal 28 may also optionally include two components, a display signalrouted to a display 30A and a feedback or control signal routed to ahydraulic control 30B.

As another alternative, the output signal 28 may comprise positionalinformation that is passed to a control box 30C. At the control box 30C,the output signal 28 is communicably coupled to a control unit 34 thatpasses a suitable signal to a control box display 36 to display theoutput signal 28. The control unit 34 further generates an appropriatefeedback control signal, which is coupled to control one or morehydraulic valves 38.

As still another exemplary alternative, the signal output 28 cancomprise positional information that is communicably coupled to acontrol box 30D. The control box 30D includes a control unit 40 adaptedto translate the positional information received at the control unit 40into a suitable control feedback signal for control of one or morehydraulic valves 42 without actually providing any position informationto a display.

Referring to FIG. 3, a device block diagram 50 according to anembodiment of the present invention illustrates the programmablemeasurement filter implemented digitally. The detector 14 outputs ameasurement signal 24 that is communicably coupled to the signalprocessing unit 26. The measurement signal 24 may be optionallyconditioned at signal conditioning box 52. Signal conditioning mayinclude any arrangement of gain adjustment, buffering, current orvoltage conversion, or other typical processing. The measurement signalis further converted to a digital format via the analog to digitalconverter 54 prior to being processed and filtered by the signalprocessing unit 26. The signal processing unit 26 includes a processor56, memory 58 adapted to store at least operational instructions andprogram code to execute the dynamically programmable digital measurementfilter 60.

The signal processing unit 26 performs necessary processing andfiltering, then provides an output signal 28 to the output device 30 aspreviously described. The signal processing unit 26 may comprise acentral processing unit such as a microprocessor, or may be implementedfor example, using specialized digital chips such as field programmablegate arrays, dedicated digital signal processing chips or other digitalarchitectures.

The Programmable Measurement Filter

The signal processing unit 26 is operatively configured to analyze themeasurement signal 24 and other available data and determine at leastone filter parameter value therefrom. The filter parameter value maycomprise a suitable value for any parameter associated with the type ofmeasurement filter implemented. For example, a filter parameter valuemay comprise a filter constant for a digital filter implemented as adifference equation. A filter parameter value may also include forexample, a cutoff frequency, bandwidth, Q-factor, gain, or a controlsignal adapted to affect a performance characteristic of the measurementfilter. The signal processing unit 26 may also control multipleparameters of the measurement filter. For example, if the measurementfilter is implemented as a Kalman filter, the filter parameter value maycomprise the Q and/or R parameters.

According to an embodiment of the present invention, the signalprocessing unit 26 attempts to determine from the measurement signal 24whether the measurement signal comprises “true” device movement, orwhether the measurement contains noise. The signal processing unit 26assigns a filter parameter a first predetermined value for detectedmovement, and a second predetermined value for noise. For example, theprogrammable measurement filter filters the measurement signal with afirst bandwidth that is dynamically programmed by the signal processingunit when device movement is detected and the signal processing unitprograms the measurement filter to operate at a second bandwidth whenonly noise is detected.

Referring to FIG. 4, a method 100 is provided for automatically tuning aprogrammable measurement filter. Initially, a measurement signal isreceived at 102. A determination is made as to whether the inputmeasurement comprises noise or true device movement at 104. If themeasurement signal is determined to be noise, the measurement signal isfiltered using first filter characteristics at 106. For example, themeasurement signal may be filtered at a relatively lowbandwidth/relatively slower responsiveness to provide greater stabilityduring the noisy conditions. If a true device movement is detected, themeasurement signal is filtered using second filter characteristics at108 where the second filter characteristics are different from the firstfilter characteristics. For example, the second filter characteristicsmay define a second bandwidth having higher/quicker responsiveness thanthe first bandwidth. The filtered measurement signal is then optionallyprocessed at 110 to format the output signal into a suitable format andis output, for example, into a suitable display signal or a suitablecontrol or feedback signal at 112. The signal processing unit variablyprograms the measurement filter to operate at a select one of the firstor second filter characteristics to dynamically respond to changes inthe measurement signal.

Where the programmable measurement filter is implemented digitally, itmay be advantageous to accomplish the filtering in a computationallycost effective manner. For example, according to an embodiment of thepresent invention, the programmable measurement filter is digitallyconstructed using a two-element infinite impulse response (IIR) filter.The automated programmable nature of the present invention allows theprogrammable measurement filter to outperform moving window average andother current filter configurations that require between 2 and 4 timesthe memory locations for performing filtering.

Any variety of digital filtering may be used depending upon factors suchas desired filtering smoothness, induced lag, and frequency responserequired of the output signal. For example, according to an embodimentof the present invention, measurement filter may be based upon adifference equation expressed generally as:y(n)=α*y(n−1)+(1−α)x(n)where α=e^(−2πf) ^(c) ^(T), f_(c) is the desired filter cutoff, and T isthe sampling period of the system, which can be a measured or pre-setvalue.

For example, a laser device expecting to receive a 10 Hz laser signal isimplemented with a dynamically tunable IIR lowpass filter that selects arelatively higher bandwidth, such as a filter having a cutoff frequencyof approximately 1.1 Hz represented by the difference equation:y(n)=0.5*y(n−1)+0.5*x(n)when true device movement is detected, and a lower frequency bandwidth,such as a filter having a cutoff frequency of approximately 0.46 Hzrepresented by the equation:y(n)=0.75*y(n−1)+0.25*x(n)when the measurement signal comprises noise. In this case, thedifference equations for the 1.1 Hz cutoff frequency and the 0.46 Hzcutoff frequency look similar with the exception of the filter constantα. Accordingly, the signal processing unit can change the frequency ofthe programmable measurement filter merely by replacing the filterconstant α in the difference equation to a value or either 0.5 or 0.75.Such may be accomplished by merely writing the filter constant value toa memory location. The coefficients are easily implemented with bitshifts implying factors of 2 scaling. To implement the measurementfilter as a second order filter, the measurement signal can be processedby the difference equation twice.

Referring back to FIG. 3, the programmable measurement filter accordingto another embodiment of the present invention comprises a filterdesigned to have a filter bandwidth minimum, a filter bandwidth maximum,and optionally one or more intermediate filter bandwidths. The signalprocessing unit 26 assigns a filter parameter value that is used toselect the desired filter bandwidth from within the operational range offilter bandwidths based upon any number of conditions. For example, thefilter bandwidth may be dynamically programmed to reflect the likelihoodthat the measurement signal 24 comprises true device movement.

For example, the digitally implemented measurement filter discussedabove may also be designed to have one or more intermediate bandwidths.For example, the filter constant can be selected in the range0.5≦α≦0.75. Referring to FIG. 5, a method 150 is provided fordynamically tuning the measurement filter. A variance σ of the noiseassociated with the measurements of interest is optionally determined at152. For example, the variance σ may be determined a priori and may betailored to specific applications or circumstances. An upper and lowerbounds Tmax and Tmin may also be optionally identified at 154. The upperand lower bounds Tmax and Tmin preferably take into account the varianceσ of the noise associated with the measurements of interest and may alsobe affected by other factors such as acceptable measures of lag versusaccuracy of measurement and operating frequency of the device.

A measurement signal is received at 156, an attempt is made todiscriminate device movement compared to noise at 158 and theprogrammable measurement filter is tuned at 106. Any number oftechniques may be used to distinguish actual or true movement fromnoise. For example, according to an embodiment of the present invention,an attempt is made to determine how likely it is that a true movement isor is not taking place at 158. A processor determines the probabilitythat true movement is or is not occurring. The measurement signal isthen filtered, preferably within the upper and lower bounds determinedat 154, based upon the probability that true device movement hasoccurred at 160. For example, the signal processing unit can modify theoperational characteristics of the programmable measurement filter tobehave approaching Tmax as the likelihood of true movement increases.Likewise, the operational characteristics of the measurement filter maybe tuned approaching Tmin as the likelihood of noise increases.

Discriminating Device Movement From Noise

As pointed out above, any number of approaches can be used todiscriminate movement from noise. For example, referring to FIG. 6, amethod 170 for automatically tuning a measurement filter according to anembodiment of the present invention is illustrated.

A measurement signal is filtered using first filter characteristics toproduce a first filtered output at 172. The measurement signal is alsofiltered using second filter characteristics to produce a second filteroutput at 176. A relationship between the first and second filteroutputs is established at 176 and a tuning signal is defined at 178based upon the established relationship between the first and secondfilter outputs. The tuning signal preferably discriminates devicemovement from noise, or alternatively, provides a likelihood that truedevice movement is or is not occurring. Measurement filtercharacteristics are determined based upon the tuning signal at 180 andthe measurement filter is tuned at 182 based upon the determinedmeasurement filter characteristics.

The measurement filter output can be formatted for a display device, orfor a control application. Further, the tuning signal can be used as afeedback control for automated control applications. Still further, boththe tuning signal and the measurement filter output can be used asfeedback signals for control applications.

Referring to FIG. 7, a block diagram of a system 200 is provided forautomatically tuning the programmable measurement filter according tothe method 170. A measurement signal 202, such as from a detector, isinput in parallel to a first filter 204 having a first bandwidth and toa second filter 206 having a second bandwidth different from the firstbandwidth.

The first and second filters 204, 206 may comprise first or higher orderfilters. According to an embodiment of the present invention, the firstfilter 204 comprises a median filter. A median filter has the capabilityof preserving discontinuities of the input signal, while at the sametime eliminating ‘flutter’ about the core energy of the signal. Themedian filter can provide an effective filter even with a small numberof samples. For example, the median filter may only look at threesamples. Compared to a 2-pole lowpass filter with a relatively lowcutoff frequency, the median filter does not have significant lag. Themedian filter output does not necessarily trend to the true mean of thesignal, but the median filter output may trend near the mean assumingthat the noise is fairly random and symmetric, and that the resolutionof the system is adequate.

According to an embodiment of the present invention, the second filter206 is constructed as a lowpass filter. A lowpass filter such as a2-pole lowpass filter with a low cutoff can be designed to preserve andapproach the mean value of the signal. The lower the output bandwidth,the better the lowpass filter is at detecting the mean over time.However, the cost associated with the precision of detecting the mean isa time lag associated with the lowpass filter output.

If the device undergoes movement relative to a transmitter, such as avertical displacement, the median filter will detect the movement in atime equal to half of the median window width. The filter such as asecond order lowpass filter with a low cutoff will also track thevertical displacement of the device, however, the lowpass filter willtrack the vertical displacement of the device based upon an exponentialcurve and will have an output that lags behind the output of the medianfilter.

If on the other hand, the measurement signal comprises only a noisecomponent such as may occur from noise such as wind gusts, beam bounce,electronic noise and vibration, the output of the median filter willclosely match the output of the 2-pole filter with a low cutoff.

Thus the difference in the output of the first and second filters 204,206 can be used as a measure to quantify how likely it is that a ‘true’movement is or is not taking place. The larger the difference betweenthe filter output of the median filter and the lowpass filter, the morelikely it is that the input sensed by the detector comprises truemovement, thus the input is preferably filtered by the programmablemeasurement filter at a higher bandwidth for tracking movement. Thesmaller the difference between the median filter and the lowpass filter,the more likely it is that the input comprises noise and is preferablyfiltered by the programmable measurement filter at a lower bandwidth toreduce or eliminate the detected noise.

The outputs from the first and second filters 204, 206 are subtracted bythe summer 208 to produce a difference signal. The absolute valueprocessor 210 determines the magnitude of the difference signal. Theoutput from the absolute value processor 210 defines a tuning signal212. Filter parameters are derived at 214 and 216 and the filterparameters are used to tune the programmable measurement filter 218. Assuch, the tuning signal output can be used to dynamically drive thedesired filter parameter value. For example, the tuning signal outputmay be compared to threshold values or used in any number ofcomputations or lookups to determine the filter parameter valuesnecessary to suitably tune the measurement filter. The tuning signal 212may also be used as a feedback signal for control applications.

According to an embodiment of the present invention, the system 200including the programmable measurement filter is implemented insoftware. The programmable measurement filter is implemented using adifference equation to implement an infinite impulse response filterhaving a lowpass function. The tuning signal 212 is thus used to derivethe necessary filter parameter values to tune the cutoff frequency ofthe measurement filter to obtain a desired frequency response. Forexample, a difference equation such as:y(n)=α(n−1)*y(n−1)+((1−α(n−1))*x(n−1))may be digitally implemented in software where α(n)=e^(−2πf) ^(c)^((n)Ts). Also, let f_(c)(n) represent the desired filter cutoff, andT_(s) is the sampling period of the system, which can be a measured orpre-set value. Using the above difference equation to implement themeasurement filter, the cutoff frequency can be tuned by altering thefilter constant α used by the difference equation.

There are a number of ways that the filter constant α can be computed.One manner is to determine a suitable low bandwidth filter and asuitable high bandwidth filter, compute the filter parameter values foreach, and use the tuning signal to select between one of the twoavailable filter parameter values.

Another alternative is to determine a suitable low bandwidth filter anda suitable high bandwidth filter, and use the tuning signal to derivefilter parameters that result in a filter between high bandwidth and lowbandwidth filters, but also allow one or more intermediate bandwidths.The transition between the minimum or low bandwidth and maximum or highbandwidth can be linear or nonlinear. The actual frequency values forthe high bandwidth filter and low bandwidth filter will vary dependingupon the application. The frequency values may also take into accountfactors such as the variance σ of the noise associated with themeasurements of interest.

A first example is to design the tunable filter such that the filterconstant varies linearly between a minimum value and a maximum value.That is, α^(min)≦α(n)≦α^(max). Let α^(min) define a filter constant thatwhen implemented in the programmable measurement filter will produce afilter output having a predetermined highest bandwidth. As such, α^(min)will also be denoted as α^(highbandwidth). Likewise, let α^(max) definea filter constant that when implemented in the programmable measurementfilter will produce a filter output having a predetermined lowestbandwidth. As such, α^(max) will also be denoted as α^(lowbandwidth). Inthis nomenclature, α^(highbandwidth)<α^(lowbandwidth). Also, let thetuning signal, denoted T(n) be bound by a minimum and maximum such thatT^(min)≦T(n)≦T^(max). The tuning signal T(n) can have any arbitrary ormeaningful range, but is preferably associated with a variance (σ)associated with the noise of the input.

Given the above defined restraints, the filter constant α(n) can beassigned a generally linear relationship to the tuning signal T(n) bythe expressions:

${\alpha(n)} = {\frac{\begin{matrix}{{\alpha(n)} = {{\alpha^{highbandwidth}\mspace{14mu}{for}\mspace{14mu}{T(n)}} > T^{\max}}} \\{{\alpha(n)} = {{\alpha^{lowbandwidth}\mspace{14mu}{for}\mspace{14mu}{T(n)}} < T^{\min}}} \\{\left( {\alpha^{highbandwidth} - \alpha^{lowbandwidth}} \right)\left( {{T(n)} - T^{\min}} \right)}\end{matrix}}{T^{\max} - T^{\min}} + \alpha^{lowbandwidth}}$

Likewise, a frequency characteristic such as the cutoff frequency can bedesigned to vary linearly between the low bandwidth and high bandwidthlimits. Under this approach, f_(c) ^(highbandwidth)>f_(c)^(lowbandwidth) and

${f_{c}(n)} = {\frac{\begin{matrix}{{{fc}(n)} = {{{f_{c}}^{highbandwidth}\mspace{14mu}{for}\mspace{14mu}{T(n)}} > T^{\max}}} \\{{{fc}(n)} = {{{f_{c}}^{lowbandwidth}\mspace{14mu}{for}\mspace{14mu}{T(n)}} < T^{\min}}} \\{\left( {{f_{c}}^{highbandwidth} - {f_{c}}^{lowbandwidth}} \right)\left( {{T(n)} - T^{\min}} \right)}\end{matrix}}{T^{\max} - T^{\min}} + {f_{c}}^{lowbandwidth}}$

Once the cutoff frequency f_(c)(n) is computed, the filter constant α(n)can be determined.

A one pole adaptive filter response y(n) with unity dc gain can then beconstructed using the position measurement x(n) and filter constant α(n)according to the difference equation:y(n)=α(n−1)*y(n−1)+((1−α(n−1))*x(n−1)) where 0≦α≦1.In the above equation, the filter constants α and (1−α) are used topreserve unity gain operation. The signal processing unit couldalternatively compute two different filter constants α and β where β issubstituted for the filter constant (1−α) if the particular applicationso warrants, for example, where non unity gain filtering is desired.

Although the forward looking rectangular approximation of the discretetime integrator is expressed above, other approximations includingbackward looking and trapezoidal approximations may be used. Further,any order filter can be constructed depending upon the application,resources available, and the ability of the processor to afford the costassociated with the increased computational burden of more elaboratefiltering. For example, a second order filter may be realized byprocessing the measurement signal through the above difference equation,then processing the filtered measurement signal through the differenceequation a second time.

Also, although IIR filters are described herein, other filter types,examples of which may include finite impulse response filters (FIR)(with or without windowing), raised-cosine, raised root cosine, linearphase, Gaussian (lowpass), Kalman, median, mean, and averaging filtersmay be used as well. Further, although lowpass filters are discussedabove, any digital filter types such as lowpass, highpass, bandpass, ornotch filters can be implemented with an appropriate difference equationas the application dictates.

Control Applications

The various embodiments of the present invention may also be used toprovide feedback control signals for automated processes. For example,the filtered measurement signal may be used to control hydraulic valvesused to adjust the height of a blade on an earthmoving machine for gradeapplications. Under such an operation, real time control of thehydraulics is important to obtain and maintain proper grade. However,excessive phase lag caused by filtering the measurement signal may limitthe ability of the hydraulic control to quickly respond to detectedchanges in grade.

Referring to FIG. 8, a system 300 is provided for compensating for thephase lag change of the measurement filter for control applicationsaccording to an embodiment of the present invention. A measurementsignal X(n) is provided to a tunable measurement filter 302. Themeasurement filter is tuned by the tuning signal output T(n) provided bya first processor such as a tuning signal unit 304 to produce a filteredmeasurement signal output. The tuning signal unit 304 produces a tuningsignal output T(n) that dynamically changes in response to predeterminedconditions. For example, the tuning signal unit 304 may comprise asignal processing unit adapted to compute a tuning signal based upon themeasurement signal X(n) as described with reference to FIGS. 6 and 7.The filtered measurement signal output may optionally requireconditioning, such as provided by the signal conditioning unit 306 tosuitably format the filtered measurement signal output for the intendedcontrol application. At least one of the tuning signal T(n) and thefiltered measurement signal output Y(n) are provided to the controlapplication 308.

The control application 308 then uses the appropriate signals toaccommodate for phase lag. For example, the filtered measurement signaloutput Y(n) may be suitably formatted as a position signal. As such, theposition signal is designated Y(n). The control application 308 uses theposition signal Y(n) and/or the tuning signal T(n) to selectively switchamong two or more discrete control feedbacks 310. This arrangement maybe useful for example, where the control application 308 utilizesdiscrete filter types or distinct filters for each of the 1 through Ncontrol feedbacks where N is any integer greater than 1. Alternatively,the 1 through N discrete feedbacks 310 may be replaced with acontinuously variable feedback 312. Under such an arrangement, thetuning signal T(n) and/or the position signal Y(n) are used to tune thecontinuously variable feedback 312 to compensate for changes in phaselag of the system. The control application 308 is controllably coupledto one or more controllers 314 such as actuators and/or valves toperform the desired control function.

There are a number of ways that the control application 308 may use thetuning signal T(n) and/or the position signal Y(n) to control thefeedbacks to accommodate for phase lag. The output of two or morecontrollers 314 may be manipulated based upon the tuning signal T(n) orposition signal Y(n). For example, the output of two or more controllersmay be added in proportion to the selection of the tuning signal T(n)and/or position signal Y(n) used.

As another example, many current hydraulic control systems use anindustry standard proportional-integral-derivative (PID) controlalgorithm. The tuning signal T(n) and/or the position signal Y(n) areused by the control application 308 to switch one or more PID constantsto compensate for changes in phase lag of the system. For example, incertain control applications, the integral gain is not required sincethe hydraulics act as integrators. Rather, the proportional gain(P-gain) is the main driving element for the control system. The tuningsignal T(n) and/or position signal Y(n) are used by the controlapplication 308 to dynamically adjust the constant that affects theP-gain. While PID control algorithms are widely used, the presentinvention may be practiced with other control models such as feedforward compensation algorithms, intelligent learning control paradigms.Further, the present invention is not limited to hydraulic controlapplications. Rather, the present invention may be practiced with anypractical actuator technology.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

1. A device for a spatial positioning system comprising: a detectoradapted to detect a measurement signal; a signal processing unitcommunicably coupled to said detector, said signal processing unitadapted to analyze said measurement signal and determine at least onefilter parameter value therefrom, wherein said signal processing unitcompares a first filter output of a first filter to a second filteroutput of a second filter where said first and second filters have atleast one different filter characteristic to discriminate devicemovement from noise detected by said detector and said signal processingunit further comprising a programmable measurement filter adapted toreceive and filter said measurement signal based upon said at least onefilter parameter value determined by said signal processing unit; and anoutput device coupled to said signal processing unit.
 2. The deviceaccording to claim 1, wherein said signal processing unit selects afirst filter parameter value from at least two predetermined filterparameter values.
 3. The device according to claim 2, wherein saidsignal processing unit selects said first filter parameter value basedupon an attempt to discriminate device movement from noise in saidmeasurement signal.
 4. The device according to claim 2, wherein saidsignal processing unit is adapted to compute said at least one filterparameter value such that a frequency characteristic of saidprogrammable measurement filter varies linearly within a predeterminedoperational range.
 5. The device according to claim 2, wherein saidsignal processing unit is adapted to compute at least one filterparameter value that varies linearly within a predetermined operationalrange.
 6. The device according to claim 2, wherein said signalprocessing unit selects a first filter parameter value from within arange of operational values based upon a likelihood that saidmeasurement signal represents device movement compared to noise.
 7. Thedevice according to claim 1, wherein said programmable measurementfilter comprises a programmable low pass filter having a cut offfrequency determined at least in part, by said at least one filterparameter value provided by said signal processing unit.
 8. The deviceaccording to claim 7, wherein said signal processing unit selects afirst filter parameter value between at least two predetermined filtervalues.
 9. The device according to claim 8, wherein said first parametervalue is selected such that said programmable low pass filter has afirst cutoff frequency in response to detected device movement and asecond cutoff frequency different from said first cutoff frequency inresponse to detected noise.
 10. The device according to claim 7, whereinsaid signal processing unit is adapted to compute said at least onefilter parameter value such that said cutoff frequency of saidprogrammable measurement filter varies linearly within a predeterminedoperational range.
 11. The device according to claim 7, wherein saidsignal processing unit is adapted to compute at least one filterparameter value that varies linearly within a predetermined operationalrange.
 12. The device according to claim 1, wherein each of said firstand second filters processes said measurement signal.
 13. The deviceaccording to claim 1, wherein said first filter comprises a medianfilter and said second filter comprises a low pass filter.
 14. Thedevice according to claim 1, wherein said programmable measurementfilter is implemented digitally as software code and said signalprocessing unit communicates said at least one filter parameter value tosaid programmable measurement filter by storing said at least one filterparameter value in a storage location of a storage device accessible bya processor executing said software code.
 15. The device according toclaim 14, wherein said processor is a component of said signalprocessing unit.
 16. The device according to claim 1, wherein saidprogrammable measurement filter filters said measurement signal toproduce a filtered output signal that is coupled to a display device.17. The device according to claim 1, wherein said programmablemeasurement filter filters said measurement signal to produce a filteredoutput signal that is coupled to an automated control device.
 18. Thedevice according to claim 1, wherein said programmable measurementfilter filters said measurement signal to produce a filtered outputsignal that is manipulated to compute a control signal to control anautomated control device.
 19. The device according to claim 1, whereinsaid measurement signal comprises at least one of an angularmeasurement, a detected laser signal, a detected global positioningsignal, and a detected automated tracking system signal.
 20. The deviceaccording to claim 1, wherein said measurement signal is communicatedfrom said detector to said signal processing unit using a wirelesscommunications link.
 21. The device according to claim 20, wherein saidwireless communications link comprises a wireless radio link.
 22. Adevice for a spatial positioning system comprising: a detector adaptedto detect a measurement signal; a first filter communicably coupled tosaid detector and arranged to filter said measurement signal, said firstfilter having a first filter output; a second filter communicablycoupled to said detector and arranged to filter said measurement signal,said second filter having a second filter output; a signal processingunit communicably coupled to said detector, said signal processing unitadapted to analyze said measurement signal and determine at least onefilter parameter value based at least partially from an analysis of saidfirst and second filter outputs; a storage device located within saidsignal processing unit for storing said at least one filter parametervalue; a programmable measurement filter adapted to receive and filtersaid measurement signal based upon said at least one filter parametervalue retrieved from said storage device of said signal processing unit;and an output device coupled to said signal processing unit.
 23. Amethod of filtering a measurement signal in a device for a spatialpositioning system comprising: analyzing a measurement signal from adetector of said device; filtering said measurement signal by a firstfilter to produce a first filter output and by a second filter toproduce a second filter output; discriminating whether said measurementsignal comprises movement of said device or comprises noise based atleast partially on said first and second filter outputs; determining atleast one filter characteristic based upon whether said measurementsignal is determined to be noise or device movement; automaticallytuning a programmable measurement filter based upon said at least onefilter characteristic; and filtering said measurement signal using saidprogrammable measurement filter.
 24. A method of filtering a measurementsignal in a device for a spatial positioning system comprising:providing a detector adapted to detect a measurement signal;communicably coupling a first filter to said detector and arranged tofilter said measurement signal, said first filter having a first filteroutput; communicably coupling a second filter to said detector andarranged to filter said measurement signal, said second filter having asecond filter output; communicably coupling a signal processing unit tosaid detector, wherein said signal processing unit compares said firstfilter output to said second filter output where said first and secondfilters have at least one different filter characteristic todiscriminate device movement from noise detected by said detector;analyzing said first and second filter outputs to derive at least onefilter parameter value; and tuning a programmable measurement filterbased upon said at least one filter parameter value determined by saidsignal processing unit.
 25. The method according to claim 24, furthercomprising selecting said filter parameter value based upon an attemptto discriminate device movement from noise in said measurement signal.26. The method according to claim 24, further comprising determining alikelihood that said measurement signal represents device movementcompared to noise and assigning said at least one filter parameter valuebased upon said likelihood.
 27. The method according to claim 24,wherein said at least one filter parameter value is selected such that afrequency characteristic of said programmable measurement filter varieslinearly within a predetermined operational range.
 28. The methodaccording to claim 24, wherein said at least one filter parameter valuevaries linearly within a predetermined operational range.
 29. The methodaccording to claim 24, wherein said at least one parameter value isselected such that said programmable measurement filter implements a lowpass filter having a first cutoff frequency in response to detecteddevice movement and a second cutoff frequency different from said firstcutoff frequency in response to detected noise.